Micromanipulators are used in surgical lasers to precisely focus and position surgical laser beams. There are a number of problems with such devices. There is often a loss of translational alignment of the laser beam due to the long, complicated optical path through the articulated arm of the surgical laser system. Further, deviations in the path through the arm also result in loss of coincidence between the aiming beam and surgical beam. The aiming beam generally is a harmless visible beam used to indicate the impact spot of the surgical beam. Alignment and coincidence problems are thus compounded because the two beams, although propagated through the same optical path, are affected differently because of their different wavelengths. In one type of surgical laser system the surgical beam is a CO.sub.2 infrared beam; the aiming beam is a HeNe beam of visible red light. These problems have been exacerbated by the increasing demand for ever-smaller focal diameters of the surgical beam.
One attempt to address these problems uses an achromatic lens system to manipulate the two beams to the same focus by compensating for the inherent variation of the effect of the optical system on the two different wavelengths. For CO.sub.2 /HeNe lasers, salt lenses are often used. These lenses are expensive and very sensitive to humidity, so they must be operated in an airtight chamber. While this approach compensates for misalignment due to wavelength differences, it does not sufficiently address the problem of overall system alignment. Further, the physician can only adjust for minor mechanical misalignments. For more significant misalignments, equipment servicing is required and the whole system is lost to surgery until readjustment can be effected.
In another approach a virtual image of an aiming beam is used to indicate the impact spot of the surgical beam. Since there is no HeNe beam or any other aiming beam the coincidence problems are obviated. But without the real aiming beam the plane of focus cannot be determined with sufficient certainty: the focal plane can only be determined to an accuracy within the depth of field of the microscope employed in the micromanipulator, which could range up to 10 to 20 mm. Further, the virtual aiming beam does not reflect the true size of the surgical beam, so that during defocusing, to decrease power from cutting level down to cauterizing level for example, there is no accurate indication of spot size. In both approaches the systems are operable only for a specific working distance of the microscope of the micromanipulator system. If another working distance is required the micromanipulator must be changed.